Industries have many uses for visual and infrared (VIS-IR) signature materials. For example, these materials are used as part of law enforcement or military training exercises and in operations to mark targets and other objects. Additionally, they may be used to make a cloud in front of a target, to obscure the target. Many different imaging technologies are used by the military and law enforcement, so a material that gives an intense signature across a wide range of wavelengths is desirable. In many cases, the same material needs to be visible with the naked eye, near-infrared night vision goggles, mid-wave thermal sights, and/or long-wave thermal sights.
Historically, pyrotechnic compositions have been used as signature materials. However, pyrotechnic compositions are inherently energetic, and if these materials fail to initiate at the proper time, they can become a safety hazard. User might want to control the duration, temperature and/or intensity of the visual and/or the infrared signature radiation as needed for different operations or training situations. Additionally, many pyrotechnic compositions contain ingredients that are toxic or hazardous to the environment. Other signature materials, such as clouds of colored dust or chemiluminescent formulations, have been employed to address these problems. However, these solutions have limited applications as clouds of dust are only visible during daylight and chemiluminescent formulations are not bright enough for daytime visibility and provide minimal thermal signature.
Very small particles of many metals may be pyrophoric. Pyrophoric powders may also be made by reducing iron compounds such as iron oxalate or iron oxide. Another approach is to combine iron and aluminum at high temperatures to form an iron aluminide composite. The aluminum may then be removed through a caustic leaching procedure, leaving behind a high surface area pyrophoric iron. The caustic leaching procedure is commonly used to make pyrophoric foils (as described in U.S. Pat. Nos. 4,435,481, 4,895,609, 4,957,421, 5,182,078, 6,093,498, 6,193,814 and others). Powders may be formed by grinding foils, scraping pyrophoric powder off the foils, or by sintering the iron and aluminum to together to make a brick that is ground into powder prior to leaching (see, for example, U.S. Pat. No. 5,194,219).
Prior art discloses the use of tin in activated iron systems U.S. Pat. Nos. 4,435,481, 4,895,609, 4,957,421, 4,965,095, 4,970,114, 5,028,385, 5,194,219, and 5,262,466 disclose using tin, typically as SnCl2, in the caustic leaching solution used to activate the iron. As discussed in U.S. Pat. No. 6,093,498, it is believed that the tin salt in the leaching solution results in deposition of tin metal that prevents oxidation of the activated iron in the leaching solution.
Additionally, U.S. Pat. No. 6,093,498 discloses a system that uses tinplated steel as the pyrophoric substrate. In this system, the tin is believed to aid in the diffusion of aluminum into the steel. The inventors report that using tinplated steel aided in processing because the performance was much less sensitive to the leaching time. Additionally, the activated iron made from tinplated steel gave a greater energy output with a higher peak temperature and longer heat generation time than iron made from a substrate without the tinplate. Close interaction of the iron in the substrate and tin was accomplished by using tinplated steel.
U.S. Pat. No. 8,783,186 discloses adding combustible elements such as nickel or boron to the payload to increase the visible and thermal output of the signature. While igniting secondary fuels with the pyrophoric material can certainly increase the output, it also has the disadvantage that many of these materials burn at very hot temperatures, so the fire hazard or the risk of potential heat damage can be significantly increased. All of the above-identified patents are hereby incorporated herein in their entireties by reference thereto.